406 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



the tlevelopiug eggs of the Crustacea aud probably of all the Metazoa. Since we ofteu study only 

 the rapidly achieved result, the jihases of luicleiir division may be easily missed. 



Fig. 106 is a median longitudinal section of an embryo preserved a few liours later than 

 the last. This shows the thoracicabdoniinal fold aud cleft (Ab. Ab. C.) previously referred to. 

 Tlie sternal area between the latter and the stomodiTum now lies next the egg shell, but soon 

 changes its position (Fig. 125, St. A.), owing to the forward growth of the thoracic-abdominal fold. 

 Tilt' labrum, here undeveloped, soon grows backward toward the latter, helping to bend the 

 tesoi)hageal tube and probably slightly altering the position of the mouth (Fig. 125). 



The i)roctodiBum arises as a solid invagination of the epiblast, at a considerable distance behind 

 the abdominid cleft (Fig. 106, Pd.), in a stage intermediate between the embryos represented by 

 Figs. 105, 100. A transvei'se section through the point of invagination is shown in Fig. 126, Pd. 



The relation of the embryo to the rest of the egg can be seen in Fig. 108, which belongs to 

 the same species as Fig. 100. Besides the shell, which is unnaturally distended, the egg is sur- 

 rounded by a delicate embryonic membrane (Mb.). This membrane is secreted early by the super- 

 ficial eiiiblast, as shown by the fact that it does not conform with the thoracic-abdominal fold. 



Wandering cells (Fig. 108, Y. C.) become gradually less abundant, but still continue to pass 

 to the outer surface of the egg next tlie epiblast. The transition from the wandering, auireboid 

 cell to the tinttened mesoblast cell, lying close to the surface, can be best followed at this stage. 



The fully developed egg nauplius (Fig. Ill) is about a week old. Embryos from the same 

 prawn vary slightly in size and in the degree of development, and also in the general character of 

 the cells. In some, the cells are larger and fewer in number, in others they are smaller and much 

 more numerous. The embryo is usually at one pole of the oblong egg. That repre.sented in Fig. 

 Ill is about one eightieth inch long, and if the entire end of the egg were .shown the drawing would 

 be nearly twice as large. 



The relation of the embryo to the whole egg can be determined from Fig. 127, where the plane 

 (/f section is through the long axis of the egg, but through the short axis of the embryo. In the 

 course of development the egg increases ajjpreciably in size and also changes its shape, at first bein^ 

 spherical, l)ut gradually becoming oblong. At this period the long axis of the embryo (using this 

 term to apply to the more obvious embryonic tissues of the egg) is parallel with the short axis of 

 the egg, while in the course of growth the embryo spreads over one side of the egg, until its long 

 iixis coincides with that of the latter. 



The optic disks have become large oval nuisses of cells which project from the surface, and 

 may now, for the first time, be appropriately called lohes. They represent the eye and eyestalk. 

 Tli<^ blocks of cells (S. O. G.) in intimate relation with the optic lobes are the ganglia of the antenn;e, 

 and represent a large part of the future brain. The ai)pendages are all simple, but a bud soon 

 grows out from the irosterior sides of the second pair of antenuiii. 



The right antenna is already bifid near its tip. A little later it has the appearance shown in 

 Figs. 109 and 110. Rudiments of the first pair of nnxxilhe (Mx. I.) are also present. The ganglia 

 of the second pair of antennie are developed in close union with the ganglia of the antennules. 

 Together they form the supra-a^sophageal ganglion or "br.ain." The stomodanun (Std.) appears 

 from the surface as a distinct mass of cells extending behind the labrum (Lb.). 



The thoracic abdominal Iblil, at first vertical to the surface, bends up and grows forward 

 toward the labrum, an<l a shallow groove which marks the median notch of the telson plate (Fig. 

 110) is formed at its (extremity. The anus passes forward (backward in a morphological sense) 

 from its dorsal position through the median notch until, at a considerably later period, it comes to 

 lie on the ventral side of the fold near its apex. The mass of cells (H.) behind the atuis probably 

 represents the heart. Near the mandibles and maxilhv, cells are seen with large granular nuclei. 

 These arc cells which have migrated from the yolk to this part of the embryo. Nuclei of epithelial 

 cells are sprinkled over the entire surftice of the egg, but increase in number as we approach the 

 embryo. 



The section through the entire egg (Fig. 127) shows someof the general characteristics of the egg- 

 nanplius. The thoracic abdominal fold is here cut on a level with the anus, aud lies in a wide shallow 

 groove. The yolk is composed of irregular blocks, only a few of which are represented, and con- 

 tains numerous wandering cells. Some of these (Y. C.^) are settling down on the ventral nervous 



